In the ninth century, the Doctor and Clara are captured by Vikings and taken to their village. No sooner have they arrived, however, than an enormous face materialises in the sky, claiming to be the god Odin. He collects the strongest men of the settlement -- inadvertently taking Clara and an unusual girl called Ashildr, too. They discover that their captor is actually the leader of the bloodthirsty Mire, a race which harvests the testosterone from warriors. When Ashildr declares war upon the Mire, a reluctant Doctor finds himself trying to prepare the outmatched Viking survivors for battle against an indomitable alien army.

Jamie Mathieson had scripted both Mummy On The Orient Express and Flatline for Doctor Who's thirty-fourth season, and was invited to pitch ideas for Season Thirty-Five. Unfortunately, several of his suggestions mirrored elements of Under The Lake / Before The Flood, which Toby Whithouse was already in the process of writing. Another proposal, involving the shape-shifting Zygons, was rejected because the aliens were already earmarked for a mid-year return in the serial that became The Zygon Invasion / The Zygon Inversion. However, executive producer Steven Moffat had been keen to develop a story in which the Doctor encountered Vikings -- a concept not explored on-screen since The Time Meddler in 1965 -- and this notion was offered to Mathieson as a starting point.

Mathieson devised a storyline entitled “Valkyrie”. The title referred to the supernatural warrior women of Norse mythology who guided the spirits of those slain in battle to the afterlife in Valhalla; they were often depicted as riding winged horses. The narrative began with the Doctor taking Clara to visit the island of Lindisfarne, the site of an infamous Viking raid in 793. There they wound up being captured by Vikings, and Clara and the women of Lindisfarne were taken away to the floating city of Valhalla by Valkyries who served the god Odin. Clara was brainwashed into becoming a Valkyrie, while the Doctor persuaded the Vikings to help him when their longship was attacked by another servant of Odin's, the sea monster Leviathan. The Doctor realised that the Valkyries' flying horses used anti-gravity technology, and he modified it so that he and the Vikings could attack Valhalla using Leviathan. Odin was eaten by the monster, and Clara and the other women were rescued.

The Doctor having to deal with a group of willing but inept Vikings evoked memories of Dad's Army

“Valkyrie” incorporated two elements which Moffat intended would have implications later in the season. Odin's reason for kidnapping the women of Lindisfarne was to establish a breeding program with which he aimed to produce a hybrid creature. Clara also befriended a woman who would gain immortality and become a recurring character throughout the year's remaining adventures. However, Moffat felt that “Valkyrie” needed to be refocussed, and he identified the Doctor's interactions with the Vikings as its strongest component. In particular, he liked the idea of the Doctor having to deal with a group of willing but inept Vikings, who evoked memories of the protagonists of Dad's Army, a sitcom set in a small English village during the Second World War.

Consequently, Mathieson developed a new version of the storyline, given the slightly agrammatical title “The Allfathers Army” in reference to an epithet commonly applied to Odin. He took inspiration from Eighties adventure shows like The A-Team and MacGyver, in which the heroes were often forced to use everyday items in clever ways to save the day. Mathieson also thought in terms of The Magnificent Seven, the 1960 Western classic in which a small group of elite gunfighters protected a Mexican village against bandits.

The Lindisfarne element was dropped at this stage, while the Valkyries were now the alien Mire in disguise. The Valkyries' captives were the fittest men and women from a Viking settlement, since Odin's goal was to capture strong physical specimens to swell the Mire's ranks. The Doctor challenged Odin, drawing the Mire into combat in the Viking village. He used various forms of trickery to defeat the Mire, and then saved the kidnapped townsfolk from the floating city by affixing the Mire's anti-gravity technology to a Viking longship. The only casualty amongst the humans was the settlement's armourer, Ashildr, whom the Doctor revived with a Mire device that made her immortal. This version of “The Allfathers Army” replaced Clara with an alternative companion, whom Moffat was developing in the event that Jenna Coleman left Doctor Who after the 2014 holiday special, Last Christmas.

Mathieson continued to refine “The Allfathers Army” during the summer of 2014. Instead of an armourer, Ashildr became the woman responsible for applying the Vikings' war paint; she replaced an earlier character called Tola, whose name was given to Lofty's wife. It was then decided that Ashildr should be isolated in some way from the other Vikings, perhaps because she had suffered the death of a child or because she was the victim of an abusive spouse. The companion -- who would ultimately be Clara after all, following Coleman's decision to stay for Season Thirty-Five -- would help inspire Ashildr's sense of independence and self-worth. The Mire's armour had been established as playing a role in both the brainwashing of its occupant, and the unnatural extension of the wearer's lifespan. As such, part of the Doctor's plan to defeat the Mire was to use magnets to separate them from their helmets, thereby breaking the mental conditioning and causing the oldest to crumble to dust. Odin's true form was revealed to be a small, pathetic creature.

In the first draft script, the Mire wore full-face masks of the type found in 1939 at Sutton Hoo

Mathieson's first draft script was completed in mid-October, and bore the title “Ragnarok”. Ashildr was now Nollarr's wife, and she allowed him to pose as the village's carpenter despite the fact that she was the one who actually did the work. The Mire wore full-face masks of the type found in 1939 at Sutton Hoo in Suffolk. Their helmets were horned -- as were the aliens who wore them -- because Mathieson knew that this was a common myth about Viking armour, and he was amused by the notion that the misconception might have arisen from the Mire's appearance. Clara, but not Ashildr, was taken to Valhalla, where she discovered that the Vikings' corpses were being mined for sustenance consumed by the Mire. The Valkyries still appeared there, but were revealed to be holograms, and Clara escaped by parachuting back to the ground. The Doctor defeated Odin by freeing the Mire from his control.

“Ragnarok” continued to undergo changes as the calendar turned over to 2015. At one point, Ashildr's immortality came about when the Doctor took her to the Sisterhood of Karn -- who would be reintroduced in the season premiere, The Magician's Apprentice -- where she was allowed to drink the Elixir of Life. However, it was felt that this extended detour hurt the pacing of the story's closing minutes, and the use of Mire technology was deemed more expedient; the cause of Ashildr's longevity eventually became the battlefield medical kit.

Meanwhile, there had been discussion about potential actresses to play Ashildr. A prime candidate was Maisie Williams, who was well-established for her starring role in the fantasy drama Game Of Thrones, but who had also impressed executive producer Brian Minchin with her performance in the telefilm Cyberbully. When Williams accepted the role, it was agreed that Ashildr's involvement in the narrative should be both strengthened and made more appropriate to the seventeen-year-old actress. Consequently, a somewhat younger Ashildr would now be Einarr's daughter rather than Nollarr's wife; she would accompany Clara to the Mire ship and be responsible for challenging Odin. Amongst other late changes, the Mire armour was reimagined as being much more industrial in appearance.

For Season Thirty-Five, Moffat wanted to place a renewed importance on two-part stories, which had seen a decreased prominence in recent years. However, he also wanted to link episodes together in ways that would blur the lines between distinct adventures. To this end, writer Catherine Tregenna was developing a follow-up story which would revisit Ashildr several centuries after the events of Mathieson's script. To make an explicit connection between the two episodes, they were given parallel titles: “Ragnarok” became The Girl Who Died, while Tregenna's script would be called The Woman Who Lived. They would be the fifth and sixth episodes in both the recording and broadcast orders, forming Block Three of the production schedule.

Brian Blessed withdrew from the role of Odin after collapsing on stage during a performance of King Lear

The director for The Girl Who Died was Doctor Who newcomer Ed Bazalgette. In addition to Williams, a key member of the supporting cast was intended to be Brian Blessed, playing Odin. The stentorian veteran actor was well-known from programmes such as Z Cars, and he had previously played King Yrcanos in 1986's The Trial Of A Time Lord (Segment Two). Unfortunately, Blessed was diagnosed with a heart condition after collapsing on stage during a January performance of King Lear. As a result, he withdrew from The Girl Who Died and was replaced by David Schofield, who had been in shows like Da Vinci's Demons and movies such as Gladiator.

Filming for The Girl Who Died began on March 25th at Fforest Fawr in Taff's Well, where the TARDIS materialised. Recording on the 26th took place at Roath Lock Studios in Cardiff, with Bazalgette initially shooting the sequence of Clara floating in space. He then moved over to the standing TARDIS set, where work continued on the 27th. After the weekend, action in the corridor of the Mire spaceship was taped at Roath Lock on March 30th. The same day, Williams' involvement with Doctor Who was announced by the BBC, although the nature of her character was not divulged.

The schedule for the remainder of the week was devoted solely to The Woman Who Lived, after which the cast and crew were given a week off post-Easter. As such, production on The Girl Who Died didn't resume until April 16th and 17th, when scenes in the Vikings' great hall were filmed at Roath Lock. More of this material was completed following the weekend, on the 20th and 21st. The major location for the episode was Cosmeston Medieval Village near Penarth. A recreation of an authentic fourteenth-century settlement, it had previously been visited for Season Thirty-Four's Robot Of Sherwood, and would now represent the exterior of the Viking village. Bazalgette's team was at work there from April 22nd to 24th before pausing for the weekend.

It was back to Cosmeston on April 27th and 28th, when one of the replica buildings also posed as the boathouse. Part of the latter day was spent recording pick-up shots at Roath Lock. Another Viking location which was originally to have been represented on location was Ashildr's hut. In the event, however, it was decided that part of the great hall would be repurposed for the hut, with the corresponding material taped on the 29th. On May 4th, Uskmouth Power Station in Newport provided a setting suitable for the Mire processing area; green screen shots of Odin's face appearing in the sky were also performed there. Finally, on May 6th, the concluding sequence depicting the ageless Ashildr was filmed at Roath Lock using a turntable. Various inserts were completed the same day.

One deletion from The Girl Who Died saw the Doctor discover evidence in the ageing scrolls kept by Ashildr that the Mire had visited the village many generations ago. This prompted speculation that they may have been responsible for -- or at least had an influence upon -- the Norse religion. Also lost was a reference to Lofty's baby being called Brot, which was actually the nickname Mathieson had given his brother. The Girl Who Died aired on October 17th, with its 8.20pm start time coming five minutes earlier than Under The Lake and Before The Flood.

• Doctor Who: The Complete History #81, 2018, “Story 256: The Girl Who Died”, edited by John Ainsworth, Hachette Partworks Ltd.